Portable electronic devices usually include keyboards or keypads so that information may be input. When portable electronic devices are used in the dark, the keys need to be illuminated. Each individual key may be illuminated by a respective light source located directly behind the key. This arrangement is, however, very expensive since a separate light source is required for each key. Also, without carefully designed diffusers in or behind the keys, unsightly bright spots may be seen through the keys coinciding with the light source.
A more economical approach utilizes fewer light sources with the light being directed toward the keys with the aid of a light guide in the form of a transparent plate. A known light guide includes an array of apertures. A portion of each key depends or extends into the corresponding aperture so that light can be directed into the key from the light guide. Thus, some of the light is directed toward the apertures and so illuminates the keys. A drawback with this arrangement is that it does not give uniform illumination, because the lighting effect deteriorates toward the edges of the keyboard or keypad and other locations further away from the light sources.
Referring to FIG. 5, a conventional keypad light guide 50 is shown. The light guide 50 is for a mobile phone, and defines a regular array of apertures 52 which correspond to shaped keys 53. Four prismatic indentations 58 are defined among the apertures 52. Each lens 57 is disposed under each indentation 58 in one side of the light guide 50, and is formed integrally with the light guide 50. Light is introduced into the light guide 50 from distributed light sources (not shown) such as light emitting diodes (LEDs) under each corresponding prismatic indentation 58. Each lens 57 serves to collimate the majority of light from its associated light source into a light beam through the light guide 50. The indentations 58 are arranged for reflecting light in a localized path substantially surrounding an area of each shaped key 53.
Although the design of the light guide 50 may illuminate the keys 53, the light guide 50 requires more than one light source that is an added cost and consumes more power. In addition, brightness areas may appear when the light sources are disposed adjacent to the light guide 50, thereby the optical uniformity of the light guide 50 is decreased.
What is needed, therefore, is a new light guide which can provide a desired uniform illumination by means of fewer light sources, and can improve optical uniformity by eliminating the bright areas adjacent to light sources.